One challenge facing a postlingually-deafened adult cochlear implant (CI) user is the possibility that there may be a frequency mismatch between the incoming acoustic signal and the characteristic frequency of the neurons stimulated by the implant. While listeners can clearly adapt to frequency mismatches, there is a lack of information that can allow us to separate the adaptation to frequency mismatches from other distortions that CI users face. In the proposed experiments, we address this issue by examining CI users who have residual hearing in the contralateral ear. Given that frequency mismatches are often heard perceptually as a change in the pitch of the signal relative to the representation stored in long-term memory, we plan to compare the pitch percepts elicited by electrical stimulation with those from the acoustic hearing in the contralateral ear, and observe whether those percepts change over time. Our assumption is that such changes in electrical pitch perception indicate adaptation to a frequency mismatch. By this line of logic, we plan to address the issue of quantifying adaptation to frequency mismatch via three experiments. In the first experiment, we plan to ask CI patients who have sufficient residual hearing in the contralateral ear to match the pitch elicited by stimulation of a given electrode to the pitch elicited by a tone presented to the acoustic-hearing ear. We plan to follow these pitch matches over the first two years of device use, and determine whether changes in pitch perception are also related to changes in speech perception. In the second experiment, we plan to determine the cochlear size and the location of the electrode within the cochlea. From this, we can estimate the amount of frequency mismatch that a given CI user faces. Then, using the data obtained in Experiment 1, we will explore whether larger initial frequency mismatches are associated with larger amounts of adaptation, or with worse overall performance. Finally, in the third experiment, we plan to track electric-acoustic evoked interactions in the P1-N1-P2 complex over the first two years of device use in order to obtain an objective measure of adaptation to frequency mismatch. Taken together, these proposed experiments represent one of the first attempts to quantify the amount of frequency mismatch a given patient has, and the extent to which they are able to adapt to that mismatch. As such, they address a significant lack of knowledge in the cochlear implant field. More importantly, the information gained from these proposed projects has the potential to directly shape future fitting of cochlear implants, and may have an effect on patient care. Here, we propose the use of novel techniques to address this key issue, and as such, we believe that the proposed research has a strong translational component that may ultimately benefit public health for hearing-impaired populations.